US2947522A - Recuperators - Google Patents

Description

J. D. KELLER Aug. 2, 1960 RECUPERATORS 3 Sheets-Sheet 1 Filed May 12, 1955 L VINV'ENTOR.

Ill I'll 1960 J. D. KELLER 2,947,522

RECUPERATORS Filed May 12, 1955 :5 Sheets-Sheet s r 2 F m I RECUPERATGRS John Donald Keller, 1019 Farmers Bank Bldg, Pittsburgh, Pa.

Filed May 12, 1955, Ser. No. 507,790

6 Claims. (Cl. 257309) In metallic recuperators, it is often necessary to maintain the metal walls through which heat is transmitted at a temperature which is undesirably close to the maximum which the metal can withstand for a period of time without failure. To be economically feasible, recuperators must be designed with a high rate of heat transfer. per square foot of heat-transmitting surface. The metal temperature at the designed rate of heat transmission is kept below the danger point by passing the air to be heated at a fairly high velocity over the surface on the other side of the wall from the surface in contact with the hot gases; wiping the heat from the wall surface by convection.

However, furnaces cannot at all times be operated at the maximum designed rate. When turned down to, for example, half load, the air quantity is reduced accordingly, the air velocity in the recuperator decreases, and the wiping-off of heat from the metal becomes much less effective. At the same time in most furnaces, the temperature of the hot gases entering the recuperator remains just as high as at full load. Burning-out of the tubes or other heat-transferring elements is therefore much more frequently caused by fractional-load than by full-load operation, unless special means are used to avoid it.

It is an object of this invention to prevent this overheating of the metal recuperator surfaces by maintaining at all times the full, or nearly the full rate of air flow in contact with said surfaces in that part of the recuperator which is in contact with the hottest gases, regardless of the turn-down or fractional-load operation.

It is a further object of this invention to utilize the recuperator heat-transmitting surface so effectively as to obtain the designed air temperature with a minimum area of said surface, while maintaining the metal temperature below the danger point at all times.

It is another object of this invention to obtain in a single recuperator the advantages of the counterflow or crosscounterflow type as regards efficiency and of the parallel flow or cross-parallel flow type as regards protection of the metal against excessive temperatures.

It is still another object of this invention to effect protection of the metal surfaces against overheating automatically and without dependence on the attention of the furnace operators.

Other objects of the invention will become apparent from the following description.

A recuperator is defined as a continuous-flow heat exchanger in which a part of the heat in the waste gases from a furnace is salvaged and is transmitted by conduction through a wall to the air which is to be used for combustion in the furnace. My invention however is not limited to recuperators but is applicable to any heat exchanger in which the operating temperature of the heatconducting wall is undesirably close to the maximum temperature which the material will withstand for a reasonable length of time.

The invention will become more apparent from a con sideration of the accompanying drawings, constituting a Patented Aug. 2., 1960 part hereof, in which like reference characters designate like parts and in which- Figure 1 is a vertical longitudinal sectional view through a tubular hanging-loop recuperator;

Figure 2 is a vertical longitudinal sectional view through the rear end of a tubular recuperator having several separate elements through wihch the air flows in parallel;

Figure 3 is a cross-sectional view through the same;

Figure 4 is a vertical sectional view through a stacktype recuperator.

The waste gases coming from the furnace enter at 1, pass along the heat-absorbing elements or surfaces from right to left as shown by arrows, and leave at 2 to go to the stack (not shown). In my invention, the heat-transmitting surface is divided into three parts or passes designated 44, 5-5 and 6-6. The cold air supplied by the fan or blower 3 to manifold 9* is also divided, part of it going through duct 10 to the hot end of the recuperator, or the part in contact with the hottest gases, as 4-4; while the remainder of the cold air goes through duct 11 to that part of the recuperator which is in contact with the coolest gases, as 66.

In general, the part of the air in the first air pass, as 4-4 in Fig. 1, is made to flow in approximately the same direction as the gases, or in parallel or cross-parallel flow. The remainder of the air, which goes to the second air pass, as 6-6 in Fig. l, is made to flow in approximately the opposite direction to the flow of the gases, or in counterfiow or cross-counterflow. p

The two streams of air leaving these passes, at 7 and 8 respectively, are then joined in manifold 8a and the combined stream is made to flow through the third air pass, 5-5 in Fig. 1, in approximately the opposite direction to the flow of the gases, that is, in counterfiow or crosscounterflow. The finally heated air then passes out through duct 22 to the burners of the furnace.

In the duct 11 carrying cold air from the blower 3 to the end of the recuperator having the lower temperature through, air-pass 6-6, a valve 12 is provided for reducing the quantity of air supplied to this part of the recuperator when the furnace is operated at less than maximum, or nearly maximum load.

The automatic control of valve 12 may be made to depend on either the temperature of the metal wall of pass 4--4, or the temperature of the air leaving air-pass 44, or on the rate of air flow through this first pass, or on the pressure at the burners. Preferably, control of valve 12 is effected by the rate of air flow, and preferably I utilize the first air pass 4-4 of the recuperator as its own flowmeter, for example by the means shown in Figs. 1 and 4,

wherein 13 is a diaphragm one side of which is exposed to the pressure of the air entering the first air pass 4-4 with which the diaphram chamber communicates through connection 14 from duct 10, and on the other side diaphragrn 13 is exposed to the pressure of the air leaving this first air pass, with which the diaphragm chamber com When the burners on the furnace are turned down to less than full load, the air flow at first decreases through all parts of the recuperator. The excess of force on the right-hand side of diaphragm 13 in Fig. 1 over the force on its left-hand side, therefore decreases, and spring 16- draws the diaphragm 13 and the plunger of pilot valve 17.

to the right. This movement of the pnlot valve admits pressure air from inlet 18, through connection 19 to the left side of the piston in air cylinder 21, and at the same time allows the air in said cylinder on the right-hand side of the piston to exhaust throughconnection 20 through the "open --le"ft lr nid end er'pile't van/e 17.

The piston in air cylinder 21 ist eiebyeaneedtemhve to the right in'Fig. 1, and its inovehientparti-ally closes valve 12 induct 11, reducing-the ai'r fio'wthrou'gh pass 6'--'6 until "the full rate ereir iiow through pass of the recuperato'r is restored re the normal rate.

Valve 12 is so made that it c'ah never close entirely, but'will always permit a s all properties of -air,"say 10 percent of 'the total as anriniinuin, ti) now through "the pass 66 even at low turn-down of the furnace.

The invent-ion is of eoursehor tedto the diaphragm type of controller shown, but may he used with other types of controllers such as those having inverted bells partially immersed in liquid, for flexible bellows, or the ring balance type of new controller. Also, instead of using the first air pass of the re'cu'perator as a flownreter, an orifice-type or Venturi or other type of flowmeter may be provided in inlet duct 10 or in outlet duct 7 from said first air pass, preferably the former.

As an alternative to the system described, valve 12 may belocated in duct 10 instead of in duct 11, and the 'diaphragm control connections reversed so that the valve is normally partly "closed 'at full furnace load, and is gradually'opened wider as the total'air flow is reduced. For obvious reasons, this arrangement is inferior to the preferred form previously described. 7

It is obvious that the valve 12 may be located in the outlet duct 8 instead of the inlet duct 11 of the second air pass 66, as indicated at A in Fig. lb'y dot-dash lines; and similarly the alternative reverse-action valve may beloc'ated in outlet duct 7 instead ofinlet duet it) of the first air pass '4"4, but from the operating standpoint, location in the cold-air duct as shown in the figures is preferable. 7

While the invention is not limited to specific proportions in a pr'eferred 'ernbodinient I apportion about 17- percent of the total heat-transmitting "surface of the recuperator to the first or parallel-flow pass 4'4 about 39 percent of the total heating surface to the second air ass 6"6 which is preferably counterflo'w; and the remaining 44 percent to the middle pass or third air Spass -5 which also is preferably counterflow. I cause about 40 percent of the total air flow to pass through the first air pass 4 4 at fullload or designed rating, and about 60 percent through the second air pa'ss 6 6. I proportion the air passages 'in the various p'a'sses in'suc'h mariner as to produ'ce air velocities suitable for eneeting the required wiping-oil? of heat from the metal surfaces without necessitating excessive pressure drop of the air.

From the foregoing descri tion it will be evident that as the furnace burners are turned down and the total air fl'ow through the re'cup'e'rato'r decreases, the air new through pass at the cold end 'of the reeuperator will be reduced more and more but the airflow threugh pass 44 where the metal is exposed to the hottest gases will be maintained constant, until valve 12 is closed as far as it can go, andonly thereafter will further urn-dawn begin to decrease the airflow through'pes's 4-4.

In some recuperat'ors for constructional or other reasons a 'cross-fiow design is used, with the air flowin'gthrou'gh numerous parallel passages or paths. "Such a design is shown in Figs. 2' and 3 In applying the invention in such cases, the tubes or heating elements may be divided into three groups, just as in Fig. 1 except that instead of counterfiow or parallel flow of the air with respect 'to the gases, cross-flow prevails in all three divisions.

For controlling the air flow to'the second air pass at the cold 'end of the recuperator in such cases, I may insure better protection of the metal 'of these tubes by causing the a litzivriftti the last tubes attire left in Fig. 2 to be gradually -r'educ"e'd by erasing valve l2 fir's't, then when this has been closed as far it will go, starting to close valve 12 and only after this has been closed to its limit, starting to close valve 12 This sequential closing can be effected for example by having the air-cylinder 21 operate a multiple cam bar 23 which engages lever arms 23,, etc. on the stems of the valves 12,, 12 12 With the cam faces 23,,, 23 and 23,, offset as shown in Fig. 2, the closing of these valves in sequence is elfected. The same sequential closing can alternatively be efiected by pneumatic or electrical means.

In some cases I do not use the three passes as in Fig. 1 but make the entire recuperator like the left-hand part of Fig. 2, designated 6'"6. In such cases I usually provide manifolds or header-boxes each supplying a multiplicity of heat-transmitting "elements designated by the numeral 6 with a valve 12 in the connection to each inlet or outlet manifold except those at the hot end of the recuperator. Full airflow through the elements 5 at the hot end is then maintained, when the total air flow is reduced, by automatically closing the valves 12,, 12 and 12 in sequence beginning at the cold end and progressing to ward the hot end of the re'cuperator.

In starting up a furnace, it is often necessary to use 25 percent to 30 percent more combustion air, temporarily, than will be used by the furnace at full operating rate after it has come up to temperature. The fact that the air passing through the recuperator is at a lower temperature during starting-up, automatically provides some increase of air flow capacity, but because of the drooping characteristic curve of the types of blower ordinarily provided for supplying the air, this increase is far from suificient. If the air passages in the recuperator are proportioned correctly for the quantity of air required for normal operation at full load, then since the pressure drop increases about as the square of the velocity or quantity, a blower must be provided capableof producing about 50 percent higher pressure than would otherwise be required. This requires an excessively large and expensive motor, and means inefficient operation under normal conditions. If, on the contrary, the air passages in the recuperator are made large enough to pass the maximum volume of air required when starting up with only the normal pressure drop, then for operation after the furnace is heated up the air velocity will be too low-for good efiiciency and for proper protection of the metal.

. To avoid these difficulties, I provide'a cold air bypass 24 Figure 1, around the recup'erator, from cold air duct 10 to hot air duct 22, and in duct '24 I provide a regulating valve 25 Preferably I make duct 24 so small that even with valve 25 wide open, no more than about 25 percent to 30 percent of the rated normal maximum air volume can pass through it. e

In Fig. .1 I have shown the valve 25 as manually operated. The trouble with this 'is that the furnace operators forget to close the by-pass valve after the furnace'is up to temperature. I therefore preferably arrange to operate the valve 25 automatically, as shown in Fig. 4, for example by means of a solenoid 2 6, the lead wires 27 of which connect to a source of electric current and to a switch (not shown) operated either by the fuel demand controller I lter'hatiycly, pneumatic or hydraulic means may be used to open valve 25 whcn the demand for air reaches a set limit and to close the valve when thedemand 'falls below this limit. 7

n1 recuperators to be applied to furnaces where 't he gases are very hot, such as soaking pits, I provide anaddi tional means for protecting the metal at the hot .end of the recuperat'or, in the form of an automatically controlled bleeder valve 32 which allows additional air to flow through the hot-end pass 4 in Fig. 4 and escape to the atmosphere, if the temperature becomes too high. Preferably the control is effected by actual measurement of the temperature of the metal wall 4, as by the use of a thermocouple welded to said wall and actuating a temperature limit controller to open valve 32. In other cases it is simpler to control the temperature of the air leaving the first air pass, for example as shown in Fig. 4 by a bi-metal thermostat coil 33, which when heated partially unwinds and actuates pilot valve 34 to admit pressure air to cylinder 35 and thus open bleeder valve 32. A gravity-operated check valve 36 may be, but is not necessarily, provided to prevent flow of air to the bleeder from the other two air passes.

Many variations may be made in the details of the controls, valves and other parts without departing from the spirit of this invention.

By the use of the invention, it is possible in metallic reeuperators applied to high temperature furnaces such as soaking pits, to reduce the required area of heat-transmitting surface at least 25 percent below the area required in the parallel flow type of recuperator which has hitherto been used in such cases to insure safety of the metal. At the same time, the area is not appreciably more than in the counterflow type which has proved impractical for use on high temperature furnaces because of excessive metal temperature.

I claim:

1. In a recuperator for an industrial heating furnace, a hot gas chamber for receiving the products of combustion of said furnace having heat exchange conduits for heating air disposed in the path of travel of the hot gases passing through said chamber, said conduits being divided into a plurality of air passes each having air inlets and outlets, the conduits of one of said passes being disposed in the region of the hottest gases passing through said chamber, the conduits of another of said passes being disposed in the region of the coolest gases in said chamber, and the conduits of the other of said air passes being disposed intermediate the first and second-named passes, a source of air connected to the inlets of the conduits of said first and second-named passes, which air varim in volume with the heat demands of the furnace, means connecting the air outlets of the first and second-named passes to the air inlet of the other pass and means for maintaining air flow substantially constant in the first-named pass by varying the air flow through the second-named pass when the total air flow through the recuperator is reduced.

2. In a recuperator for an industrial heating furnace, a hot gas chamber for receiving the products of combustion of said furnace having heat exchange conduits for heating air disposed in the path of travel of the hot gases passing through said chamber, said conduits being divided into a plurality of air passes each having air inlets and outlets, the conduits of one of said passes being disposed in the region of the hottest gases passing through said chamber, the conduits of another of said passes being disposed in the region of the coolest gases in said chamher and the conduits of the other of said air passes being disposed intermediate the first and second-named passes, a source of air connected to the inlets of the conduits of said first and second-named passes, which air varies in volume with the heat demands of the furnace, means connecting the air outlets of the first and second-named passes to the air inlet of the other pass and means actuated by the air pressure drop through the first-named pass to maintain said pressure drop substantially constant by reducing the air flow through the second-named pass when the total air flow through the recuperator is reduced.

3. In a recuperator for an industrial heating furnace, a hot gas chamber for receiving the products of combustion of said furnace having heat exchange conduits for heating air disposed in the path of travel of the hot gases passing through said chamber, said conduits being divided into a plurality of air passes each having air inlets and outlets, the conduits of one of said passes being disposed in the region of the hottest gases passing through said chamber, the conduits of another of said passes being disposed in the region of the coolest gases in said chamber, and the conduits of the other of said air passes being disposed intermediate the first and second-named passes, a source of air connected to the inlets of the conduits of said first and second-named passes, which air varies in volume with the heat demands of the furnace, means connecting the air outlets of the first and second-named passes to the air inlet of the other pass and means for limiting the maximum metal temperature of the conduits in said first-named pass by reducing the air flow through the second-named pass when the total air flow through the recuperator is reduced.

4. In a recuperator for an industrial heating furnace, a hot gas chamber for receiving the products of combustion of said furnace having heat exchange conduits for heating air disposed in the path of travel of the hot gases passing through said chamber, said conduits being divided into a plurality of air passes each having air inlets and outlets, the conduits of one of said passes being disposed in the region of the hottest gases passing through said chamber, the conduits of another of said passes being disposed in the region of the coolest gases in said chamber and the conduits of the other of said air passes being disposed intermediate the first and second-named passes, a source of air connected to the inlets of the conduits of said first and second-named passes, which air varies in volume with the heat demands of the furnace, means connecting the air outlets of the first and second-named passes to the air inlet of the other pass and means for maintaining a predetermined metal temperature of the conduits in said first-named pass by automatically reducing the air flow through the second-named. pass when the total air flow through the recuperator is reduced.

5. A recuperator structure as set forth in claim 1 in which the air flow in the first-named pass is substantially in the same direction as the direction of travel of the hot gases, the air flow in the second-named pass being substantially counter to the direction of flow of the hot gases and the flow in the third-named pass being substantially counter to the direction of the hot gases flowing in said chamber.

6. A recuperator as set forth in claim 1 having a bypass connection from the air inlet of said one of said passes to the air outlet of said other of said passes and a valve in said by-pass, the by-pass and valve being of a size to limit the air flow through it when the valve is wide open to not more than 30% of the air flow through the recuperator.

References Cited in the file of this patent UNITED STATES PATENTS 1,105,798 Kennedy Aug. 4, 1914 1,599,613 Fahrenwald Sept. 14, 1926 1,673,122 Mills June 12, 1928 1,789,880 Price Jan. 20, 1931 1,964,256 Fahrenwald June 26, 1934 2,395,384 Ziebolz Feb. 19, 1946 2,499,358 Cooper et al Mar. 7, 1950 2,511,647 Marshall June 13, 1950 2,521,866 Ott Sept. 12, 1950 2,650,555 Howland et a1 Sept. 1, 1953 2,683,590 Baver July 13, 1954 2,714,847 Svebel Aug. 9, 1955 2,729,301 Ekstrom Jan. 3, 1956 FOREIGN PATENTS 232,209 Switzerland Aug. 1, 1944 430,415 Germany Dec. 20, 1920

Images